lisa/_assets/kmodules/lisa: Fix MODULE_SRC definition

The $(src) variable contains either the path to the module on its
own in out-of-tree builds or the path relative to $(srctree) in in-tree
builds. Fix the definition of MODULE_SRC to account for that
discrepancy.

Signed-off-by: Kajetan Puchalski <kajetan.puchalski@arm.com>